Power drive units for aircraft cargo handling systems conventionally include a drive roller coupled to an electric motor for engaging and propelling cargo on a ball or roller conveyor surface to and from storage positions within an aircraft hull. When cargo becomes stacked or jammed, continued operation of the drive roller scrubs the outer surface of the roller against the cargo engaged by the roller. Such scrubbing wears the friction surface of the drive roller and wastes electrical energy at the motor. It is therefore one object of the present invention to provide a mechanism for sensing and preventing scrubbing between the drive roller and cargo in aircraft handling systems of the subject character.
Self-erecting power drive units are particularly advantageous in many aircraft cargo handling system applications. In these units, application of electrical power to the drive motor initially operates through cams that engage a fixed mechanism to raise the drive unit from a retracted position beneath the conveyor surface to an erected position in which the drive roller is disposed at the cargo conveyor plane for engagement with the cargo units. Another object of the present invention is to provide an improved power splitting planetary gear differential between the motor on the one hand, and the self-erecting mechanism and drive roller on the other, that includes an improved torque control mechanism of reduced weight for splitting power between the self-erecting mechanism and the drive roller, that has no wear surfaces, and that does not employ a fluid that can overheat or leak.
A power drive unit for aircraft cargo handling systems in accordance with the present invention includes an electric motor and a drive roller mounted to rotate about an axis parallel to the motor output shaft and carried for engagement with cargo to be driven by the unit. A gear transmission couples the motor output shaft to the drive roller shaft for rotating the drive roller responsive to application of electrical power to the motor. A scrub sensor modulates application of electrical power to the motor when the drive roller is scrubbing against cargo engaged by the drive roller. The scrub sensor in the preferred embodiments of the invention includes a second roller mounted for free rotation adjacent to the drive roller and having a periphery disposed for engagement with cargo engaged by the drive roller. Application of electrical power to the motor is modulated as a function of rotation of the second roller.
In a self-erecting power drive unit in accordance with the preferred embodiments of the invention, the gear transmission comprises a power-splitting planetary gear differential having an input coupled to the motor output shaft, a first output coupled to a mechanism for erecting the unit from a retracted position to an erected position in which the drive roller is positioned to engage cargo, and a second output coupled to the drive roller. Torque applied by the differential is controlled for first coupling drive torque from the motor to the self-erecting mechanism to raise the unit to the erected position, and then for coupling drive torque from the motor to the drive roller after the unit is erect. Torque control preferably is accomplished by a magnetic clutch that includes a first array of magnets rotatably coupled to the differential between the input and the second output to the erecting mechanism, and a second array of magnets disposed in fixed position adjacent to the first magnets. Magnetic forces between the first and second magnet arrays inhibit rotation of the first magnets until overcome by torque applied to the first magnet array after the unit is erect.